9 my $XS_VERSION = $VERSION;
10 $VERSION = eval $VERSION;
12 # Verify this Perl supports threads
14 if (! $Config::Config{useithreads}) {
15 die("This Perl not built to support threads\n");
18 # Complain if 'threads' is loaded after 'threads::shared'
19 if ($threads::shared::threads_shared) {
21 Warning, threads::shared has already been loaded. To
22 enable shared variables, 'use threads' must be called
23 before threads::shared or any module that uses it.
27 # Declare that we have been loaded
28 $threads::threads = 1;
32 XSLoader::load('threads', $XS_VERSION);
39 my $class = shift; # Not used
41 # Exported subroutines
42 my @EXPORT = qw(async);
45 while (my $sym = shift) {
46 if ($sym =~ /^(?:stack|exit)/i) {
47 if (defined(my $arg = shift)) {
48 if ($sym =~ /^stack/i) {
49 threads->set_stack_size($arg);
51 $threads::thread_exit_only = $arg =~ /^thread/i;
55 Carp::croak("threads: Missing argument for option: $sym");
58 } elsif ($sym =~ /^str/i) {
59 import overload ('""' => \&tid);
61 } elsif ($sym =~ /^(?::all|yield)$/) {
62 push(@EXPORT, qw(yield));
66 Carp::croak("threads: Unknown import option: $sym");
70 # Export subroutine names
71 my $caller = caller();
72 foreach my $sym (@EXPORT) {
74 *{$caller.'::'.$sym} = \&{$sym};
77 # Set stack size via environment variable
78 if (exists($ENV{'PERL5_ITHREADS_STACK_SIZE'})) {
79 threads->set_stack_size($ENV{'PERL5_ITHREADS_STACK_SIZE'});
86 # Exit from a thread (only)
89 my ($class, $status) = @_;
90 if (! defined($status)) {
97 Carp::croak('Usage: threads->exit(status)');
100 $class->set_thread_exit_only(1);
104 # 'Constant' args for threads->list()
106 sub threads::running { 1 }
107 sub threads::joinable { 0 }
109 # 'new' is an alias for 'create'
112 # 'async' is a function alias for the 'threads->create()' method
115 unshift(@_, 'threads');
116 # Use "goto" trick to avoid pad problems from 5.8.1 (fixed in 5.8.2)
120 # Thread object equality checking
123 '!=' => sub { ! equal(@_) },
133 threads - Perl interpreter-based threads
137 This document describes threads version 1.66
141 use threads ('yield',
142 'stack_size' => 64*4096,
143 'exit' => 'threads_only',
148 print('Thread started: ', join(' ', @args), "\n");
150 my $thr = threads->create('start_thread', 'argument');
153 threads->create(sub { print("I am a thread\n"); })->join();
155 my $thr2 = async { foreach (@files) { ... } };
157 if (my $err = $thr2->error()) {
158 warn("Thread error: $err\n");
161 # Invoke thread in list context (implicit) so it can return a list
162 my ($thr) = threads->create(sub { return (qw/a b c/); });
163 # or specify list context explicitly
164 my $thr = threads->create({'context' => 'list'},
165 sub { return (qw/a b c/); });
166 my @results = $thr->join();
170 # Get a thread's object
171 $thr = threads->self();
172 $thr = threads->object($tid);
175 $tid = threads->tid();
179 # Give other threads a chance to run
183 # Lists of non-detached threads
184 my @threads = threads->list();
185 my $thread_count = threads->list();
187 my @running = threads->list(threads::running);
188 my @joinable = threads->list(threads::joinable);
190 # Test thread objects
191 if ($thr1 == $thr2) {
195 # Manage thread stack size
196 $stack_size = threads->get_stack_size();
197 $old_size = threads->set_stack_size(32*4096);
199 # Create a thread with a specific context and stack size
200 my $thr = threads->create({ 'context' => 'list',
201 'stack_size' => 32*4096,
202 'exit' => 'thread_only' },
205 # Get thread's context
206 my $wantarray = $thr->wantarray();
208 # Check thread's state
209 if ($thr->is_running()) {
212 if ($thr->is_joinable()) {
216 # Send a signal to a thread
217 $thr->kill('SIGUSR1');
224 Perl 5.6 introduced something called interpreter threads. Interpreter threads
225 are different from I<5005threads> (the thread model of Perl 5.005) by creating
226 a new Perl interpreter per thread, and not sharing any data or state between
229 Prior to Perl 5.8, this has only been available to people embedding Perl, and
230 for emulating fork() on Windows.
232 The I<threads> API is loosely based on the old Thread.pm API. It is very
233 important to note that variables are not shared between threads, all variables
234 are by default thread local. To use shared variables one must also use
240 It is also important to note that you must enable threads by doing C<use
241 threads> as early as possible in the script itself, and that it is not
242 possible to enable threading inside an C<eval "">, C<do>, C<require>, or
243 C<use>. In particular, if you are intending to share variables with
244 L<threads::shared>, you must C<use threads> before you C<use threads::shared>.
245 (C<threads> will emit a warning if you do it the other way around.)
249 =item $thr = threads->create(FUNCTION, ARGS)
251 This will create a new thread that will begin execution with the specified
252 entry point function, and give it the I<ARGS> list as parameters. It will
253 return the corresponding threads object, or C<undef> if thread creation failed.
255 I<FUNCTION> may either be the name of a function, an anonymous subroutine, or
258 my $thr = threads->create('func_name', ...);
260 my $thr = threads->create(sub { ... }, ...);
262 my $thr = threads->create(\&func, ...);
264 The C<-E<gt>new()> method is an alias for C<-E<gt>create()>.
268 This will wait for the corresponding thread to complete its execution. When
269 the thread finishes, C<-E<gt>join()> will return the return value(s) of the
270 entry point function.
272 The context (void, scalar or list) for the return value(s) for C<-E<gt>join()>
273 is determined at the time of thread creation.
275 # Create thread in list context (implicit)
276 my ($thr1) = threads->create(sub {
277 my @results = qw(a b c);
281 my $thr1 = threads->create({'context' => 'list'},
283 my @results = qw(a b c);
286 # Retrieve list results from thread
287 my @res1 = $thr1->join();
289 # Create thread in scalar context (implicit)
290 my $thr2 = threads->create(sub {
294 # Retrieve scalar result from thread
295 my $res2 = $thr2->join();
297 # Create a thread in void context (explicit)
298 my $thr3 = threads->create({'void' => 1},
299 sub { print("Hello, world\n"); });
300 # Join the thread in void context (i.e., no return value)
303 See L</"THREAD CONTEXT"> for more details.
305 If the program exits without all threads having either been joined or
306 detached, then a warning will be issued.
308 Calling C<-E<gt>join()> or C<-E<gt>detach()> on an already joined thread will
309 cause an error to be thrown.
313 Makes the thread unjoinable, and causes any eventual return value to be
314 discarded. When the program exits, any detached threads that are still
315 running are silently terminated.
317 If the program exits without all threads having either been joined or
318 detached, then a warning will be issued.
320 Calling C<-E<gt>join()> or C<-E<gt>detach()> on an already detached thread
321 will cause an error to be thrown.
323 =item threads->detach()
325 Class method that allows a thread to detach itself.
327 =item threads->self()
329 Class method that allows a thread to obtain its own I<threads> object.
333 Returns the ID of the thread. Thread IDs are unique integers with the main
334 thread in a program being 0, and incrementing by 1 for every thread created.
338 Class method that allows a thread to obtain its own ID.
342 If you add the C<stringify> import option to your C<use threads> declaration,
343 then using a threads object in a string or a string context (e.g., as a hash
344 key) will cause its ID to be used as the value:
346 use threads qw(stringify);
348 my $thr = threads->create(...);
349 print("Thread $thr started...\n"); # Prints out: Thread 1 started...
351 =item threads->object($tid)
353 This will return the I<threads> object for the I<active> thread associated
354 with the specified thread ID. Returns C<undef> if there is no thread
355 associated with the TID, if the thread is joined or detached, if no TID is
356 specified or if the specified TID is undef.
358 =item threads->yield()
360 This is a suggestion to the OS to let this thread yield CPU time to other
361 threads. What actually happens is highly dependent upon the underlying
362 thread implementation.
364 You may do C<use threads qw(yield)>, and then just use C<yield()> in your
367 =item threads->list()
369 =item threads->list(threads::all)
371 =item threads->list(threads::running)
373 =item threads->list(threads::joinable)
375 With no arguments (or using C<threads::all>) and in a list context, returns a
376 list of all non-joined, non-detached I<threads> objects. In a scalar context,
377 returns a count of the same.
379 With a I<true> argument (using C<threads::running>), returns a list of all
380 non-joined, non-detached I<threads> objects that are still running.
382 With a I<false> argument (using C<threads::joinable>), returns a list of all
383 non-joined, non-detached I<threads> objects that have finished running (i.e.,
384 for which C<-E<gt>join()> will not I<block>).
386 =item $thr1->equal($thr2)
388 Tests if two threads objects are the same thread or not. This is overloaded
389 to the more natural forms:
391 if ($thr1 == $thr2) {
392 print("Threads are the same\n");
395 if ($thr1 != $thr2) {
396 print("Threads differ\n");
399 (Thread comparison is based on thread IDs.)
403 C<async> creates a thread to execute the block immediately following
404 it. This block is treated as an anonymous subroutine, and so must have a
405 semicolon after the closing brace. Like C<threads-E<gt>create()>, C<async>
406 returns a I<threads> object.
410 Threads are executed in an C<eval> context. This method will return C<undef>
411 if the thread terminates I<normally>. Otherwise, it returns the value of
412 C<$@> associated with the thread's execution status in its C<eval> context.
414 =item $thr->_handle()
416 This I<private> method returns the memory location of the internal thread
417 structure associated with a threads object. For Win32, this is a pointer to
418 the C<HANDLE> value returned by C<CreateThread> (i.e., C<HANDLE *>); for other
419 platforms, it is a pointer to the C<pthread_t> structure used in the
420 C<pthread_create> call (i.e., C<pthread_t *>).
422 This method is of no use for general Perl threads programming. Its intent is
423 to provide other (XS-based) thread modules with the capability to access, and
424 possibly manipulate, the underlying thread structure associated with a Perl
427 =item threads->_handle()
429 Class method that allows a thread to obtain its own I<handle>.
433 =head1 EXITING A THREAD
435 The usual method for terminating a thread is to
436 L<return()|perlfunc/"return EXPR"> from the entry point function with the
437 appropriate return value(s).
441 =item threads->exit()
443 If needed, a thread can be exited at any time by calling
444 C<threads-E<gt>exit()>. This will cause the thread to return C<undef> in a
445 scalar context, or the empty list in a list context.
447 When called from the I<main> thread, this behaves the same as C<exit(0)>.
449 =item threads->exit(status)
451 When called from a thread, this behaves like C<threads-E<gt>exit()> (i.e., the
452 exit status code is ignored).
454 When called from the I<main> thread, this behaves the same as C<exit(status)>.
458 Calling C<die()> in a thread indicates an abnormal exit for the thread. Any
459 C<$SIG{__DIE__}> handler in the thread will be called first, and then the
460 thread will exit with a warning message that will contain any arguments passed
461 in the C<die()> call.
465 Calling L<exit()|perlfunc/"exit EXPR"> inside a thread causes the whole
466 application to terminate. Because of this, the use of C<exit()> inside
467 threaded code, or in modules that might be used in threaded applications, is
468 strongly discouraged.
470 If C<exit()> really is needed, then consider using the following:
472 threads->exit() if threads->can('exit'); # Thread friendly
475 =item use threads 'exit' => 'threads_only'
477 This globally overrides the default behavior of calling C<exit()> inside a
478 thread, and effectively causes such calls to behave the same as
479 C<threads-E<gt>exit()>. In other words, with this setting, calling C<exit()>
480 causes only the thread to terminate.
482 Because of its global effect, this setting should not be used inside modules
485 The I<main> thread is unaffected by this setting.
487 =item threads->create({'exit' => 'thread_only'}, ...)
489 This overrides the default behavior of C<exit()> inside the newly created
492 =item $thr->set_thread_exit_only(boolean)
494 This can be used to change the I<exit thread only> behavior for a thread after
495 it has been created. With a I<true> argument, C<exit()> will cause only the
496 thread to exit. With a I<false> argument, C<exit()> will terminate the
499 The I<main> thread is unaffected by this call.
501 =item threads->set_thread_exit_only(boolean)
503 Class method for use inside a thread to change its own behavior for C<exit()>.
505 The I<main> thread is unaffected by this call.
511 The following boolean methods are useful in determining the I<state> of a
516 =item $thr->is_running()
518 Returns true if a thread is still running (i.e., if its entry point function
519 has not yet finished or exited).
521 =item $thr->is_joinable()
523 Returns true if the thread has finished running, is not detached and has not
524 yet been joined. In other words, the thread is ready to be joined, and a call
525 to C<$thr-E<gt>join()> will not I<block>.
527 =item $thr->is_detached()
529 Returns true if the thread has been detached.
531 =item threads->is_detached()
533 Class method that allows a thread to determine whether or not it is detached.
537 =head1 THREAD CONTEXT
539 As with subroutines, the type of value returned from a thread's entry point
540 function may be determined by the thread's I<context>: list, scalar or void.
541 The thread's context is determined at thread creation. This is necessary so
542 that the context is available to the entry point function via
543 L<wantarray()|perlfunc/"wantarray">. The thread may then specify a value of
544 the appropriate type to be returned from C<-E<gt>join()>.
546 =head2 Explicit context
548 Because thread creation and thread joining may occur in different contexts, it
549 may be desirable to state the context explicitly to the thread's entry point
550 function. This may be done by calling C<-E<gt>create()> with a hash reference
551 as the first argument:
553 my $thr = threads->create({'context' => 'list'}, \&foo);
555 my @results = $thr->join();
557 In the above, the threads object is returned to the parent thread in scalar
558 context, and the thread's entry point function C<foo> will be called in list
559 (array) context such that the parent thread can receive a list (array) from
560 the C<-E<gt>join()> call. (C<'array'> is synonymous with C<'list'>.)
562 Similarly, if you need the threads object, but your thread will not be
563 returning a value (i.e., I<void> context), you would do the following:
565 my $thr = threads->create({'context' => 'void'}, \&foo);
569 The context type may also be used as the I<key> in the hash reference followed
572 threads->create({'scalar' => 1}, \&foo);
574 my ($thr) = threads->list();
575 my $result = $thr->join();
577 =head2 Implicit context
579 If not explicitly stated, the thread's context is implied from the context
580 of the C<-E<gt>create()> call:
582 # Create thread in list context
583 my ($thr) = threads->create(...);
585 # Create thread in scalar context
586 my $thr = threads->create(...);
588 # Create thread in void context
589 threads->create(...);
591 =head2 $thr->wantarray()
593 This returns the thread's context in the same manner as
594 L<wantarray()|perlfunc/"wantarray">.
596 =head2 threads->wantarray()
598 Class method to return the current thread's context. This returns the same
599 value as running L<wantarray()|perlfunc/"wantarray"> inside the current
600 thread's entry point function.
602 =head1 THREAD STACK SIZE
604 The default per-thread stack size for different platforms varies
605 significantly, and is almost always far more than is needed for most
606 applications. On Win32, Perl's makefile explicitly sets the default stack to
607 16 MB; on most other platforms, the system default is used, which again may be
608 much larger than is needed.
610 By tuning the stack size to more accurately reflect your application's needs,
611 you may significantly reduce your application's memory usage, and increase the
612 number of simultaneously running threads.
614 Note that on Windows, address space allocation granularity is 64 KB,
615 therefore, setting the stack smaller than that on Win32 Perl will not save any
620 =item threads->get_stack_size();
622 Returns the current default per-thread stack size. The default is zero, which
623 means the system default stack size is currently in use.
625 =item $size = $thr->get_stack_size();
627 Returns the stack size for a particular thread. A return value of zero
628 indicates the system default stack size was used for the thread.
630 =item $old_size = threads->set_stack_size($new_size);
632 Sets a new default per-thread stack size, and returns the previous setting.
634 Some platforms have a minimum thread stack size. Trying to set the stack size
635 below this value will result in a warning, and the minimum stack size will be
638 Some Linux platforms have a maximum stack size. Setting too large of a stack
639 size will cause thread creation to fail.
641 If needed, C<$new_size> will be rounded up to the next multiple of the memory
642 page size (usually 4096 or 8192).
644 Threads created after the stack size is set will then either call
645 C<pthread_attr_setstacksize()> I<(for pthreads platforms)>, or supply the
646 stack size to C<CreateThread()> I<(for Win32 Perl)>.
648 (Obviously, this call does not affect any currently extant threads.)
650 =item use threads ('stack_size' => VALUE);
652 This sets the default per-thread stack size at the start of the application.
654 =item $ENV{'PERL5_ITHREADS_STACK_SIZE'}
656 The default per-thread stack size may be set at the start of the application
657 through the use of the environment variable C<PERL5_ITHREADS_STACK_SIZE>:
659 PERL5_ITHREADS_STACK_SIZE=1048576
660 export PERL5_ITHREADS_STACK_SIZE
661 perl -e'use threads; print(threads->get_stack_size(), "\n")'
663 This value overrides any C<stack_size> parameter given to C<use threads>. Its
664 primary purpose is to permit setting the per-thread stack size for legacy
665 threaded applications.
667 =item threads->create({'stack_size' => VALUE}, FUNCTION, ARGS)
669 To specify a particular stack size for any individual thread, call
670 C<-E<gt>create()> with a hash reference as the first argument:
672 my $thr = threads->create({'stack_size' => 32*4096}, \&foo, @args);
674 =item $thr2 = $thr1->create(FUNCTION, ARGS)
676 This creates a new thread (C<$thr2>) that inherits the stack size from an
677 existing thread (C<$thr1>). This is shorthand for the following:
679 my $stack_size = $thr1->get_stack_size();
680 my $thr2 = threads->create({'stack_size' => $stack_size}, FUNCTION, ARGS);
684 =head1 THREAD SIGNALLING
686 When safe signals is in effect (the default behavior - see L</"Unsafe signals">
687 for more details), then signals may be sent and acted upon by individual
692 =item $thr->kill('SIG...');
694 Sends the specified signal to the thread. Signal names and (positive) signal
695 numbers are the same as those supported by
696 L<kill()|perlfunc/"kill SIGNAL, LIST">. For example, 'SIGTERM', 'TERM' and
697 (depending on the OS) 15 are all valid arguments to C<-E<gt>kill()>.
699 Returns the thread object to allow for method chaining:
701 $thr->kill('SIG...')->join();
705 Signal handlers need to be set up in the threads for the signals they are
706 expected to act upon. Here's an example for I<cancelling> a thread:
712 # Thread 'cancellation' signal handler
713 $SIG{'KILL'} = sub { threads->exit(); };
719 my $thr = threads->create('thr_func');
723 # Signal the thread to terminate, and then detach
724 # it so that it will get cleaned up automatically
725 $thr->kill('KILL')->detach();
727 Here's another simplistic example that illustrates the use of thread
728 signalling in conjunction with a semaphore to provide rudimentary I<suspend>
729 and I<resume> capabilities:
732 use Thread::Semaphore;
738 # Thread 'suspend/resume' signal handler
740 $sema->down(); # Thread suspended
741 $sema->up(); # Thread resumes
747 # Create a semaphore and pass it to a thread
748 my $sema = Thread::Semaphore->new();
749 my $thr = threads->create('thr_func', $sema);
757 # Allow the thread to continue
760 CAVEAT: The thread signalling capability provided by this module does not
761 actually send signals via the OS. It I<emulates> signals at the Perl-level
762 such that signal handlers are called in the appropriate thread. For example,
763 sending C<$thr-E<gt>kill('STOP')> does not actually suspend a thread (or the
764 whole process), but does cause a C<$SIG{'STOP'}> handler to be called in that
765 thread (as illustrated above).
767 As such, signals that would normally not be appropriate to use in the
768 C<kill()> command (e.g., C<kill('KILL', $$)>) are okay to use with the
769 C<-E<gt>kill()> method (again, as illustrated above).
771 Correspondingly, sending a signal to a thread does not disrupt the operation
772 the thread is currently working on: The signal will be acted upon after the
773 current operation has completed. For instance, if the thread is I<stuck> on
774 an I/O call, sending it a signal will not cause the I/O call to be interrupted
775 such that the signal is acted up immediately.
777 Sending a signal to a terminated thread is ignored.
783 =item Perl exited with active threads:
785 If the program exits without all threads having either been joined or
786 detached, then this warning will be issued.
788 NOTE: If the I<main> thread exits, then this warning cannot be suppressed
789 using C<no warnings 'threads';> as suggested below.
791 =item Thread creation failed: pthread_create returned #
793 See the appropriate I<man> page for C<pthread_create> to determine the actual
794 cause for the failure.
796 =item Thread # terminated abnormally: ...
798 A thread terminated in some manner other than just returning from its entry
799 point function, or by using C<threads-E<gt>exit()>. For example, the thread
800 may have terminated because of an error, or by using C<die>.
802 =item Using minimum thread stack size of #
804 Some platforms have a minimum thread stack size. Trying to set the stack size
805 below this value will result in the above warning, and the stack size will be
808 =item Thread creation failed: pthread_attr_setstacksize(I<SIZE>) returned 22
810 The specified I<SIZE> exceeds the system's maximum stack size. Use a smaller
811 value for the stack size.
815 If needed, thread warnings can be suppressed by using:
817 no warnings 'threads';
819 in the appropriate scope.
825 =item This Perl not built to support threads
827 The particular copy of Perl that you're trying to use was not built using the
828 C<useithreads> configuration option.
830 Having threads support requires all of Perl and all of the XS modules in the
831 Perl installation to be rebuilt; it is not just a question of adding the
832 L<threads> module (i.e., threaded and non-threaded Perls are binary
835 =item Cannot change stack size of an existing thread
837 The stack size of currently extant threads cannot be changed, therefore, the
838 following results in the above error:
840 $thr->set_stack_size($size);
842 =item Cannot signal threads without safe signals
844 Safe signals must be in effect to use the C<-E<gt>kill()> signalling method.
845 See L</"Unsafe signals"> for more details.
847 =item Unrecognized signal name: ...
849 The particular copy of Perl that you're trying to use does not support the
850 specified signal being used in a C<-E<gt>kill()> call.
854 =head1 BUGS AND LIMITATIONS
856 Before you consider posting a bug report, please consult, and possibly post a
857 message to the discussion forum to see if what you've encountered is a known
862 =item Thread-safe modules
864 See L<perlmod/"Making your module threadsafe"> when creating modules that may
865 be used in threaded applications, especially if those modules use non-Perl
868 =item Using non-thread-safe modules
870 Unfortunately, you may encounter Perl modules that are not I<thread-safe>.
871 For example, they may crash the Perl interpreter during execution, or may dump
872 core on termination. Depending on the module and the requirements of your
873 application, it may be possible to work around such difficulties.
875 If the module will only be used inside a thread, you can try loading the
876 module from inside the thread entry point function using C<require> (and
877 C<import> if needed):
881 require Unsafe::Module
882 # Unsafe::Module->import(...);
887 If the module is needed inside the I<main> thread, try modifying your
888 application so that the module is loaded (again using C<require> and
889 C<-E<gt>import()>) after any threads are started, and in such a way that no
890 other threads are started afterwards.
892 If the above does not work, or is not adequate for your application, then file
893 a bug report on L<http://rt.cpan.org/Public/> against the problematic module.
895 =item Current working directory
897 On all platforms except MSWin32, the setting for the current working directory
898 is shared among all threads such that changing it in one thread (e.g., using
899 C<chdir()>) will affect all the threads in the application.
901 On MSWin32, each thread maintains its own the current working directory
904 =item Environment variables
906 Currently, on all platforms except MSWin32, all I<system> calls (e.g., using
907 C<system()> or back-ticks) made from threads use the environment variable
908 settings from the I<main> thread. In other words, changes made to C<%ENV> in
909 a thread will not be visible in I<system> calls made by that thread.
911 To work around this, set environment variables as part of the I<system> call.
915 system("FOO=$msg; echo \$FOO"); # Outputs 'hello' to STDOUT
917 On MSWin32, each thread maintains its own set of environment variables.
919 =item Parent-child threads
921 On some platforms, it might not be possible to destroy I<parent> threads while
922 there are still existing I<child> threads.
924 =item Creating threads inside special blocks
926 Creating threads inside C<BEGIN>, C<CHECK> or C<INIT> blocks should not be
927 relied upon. Depending on the Perl version and the application code, results
928 may range from success, to (apparently harmless) warnings of leaked scalar, or
929 all the way up to crashing of the Perl interpreter.
933 Since Perl 5.8.0, signals have been made safer in Perl by postponing their
934 handling until the interpreter is in a I<safe> state. See
935 L<perl58delta/"Safe Signals"> and L<perlipc/"Deferred Signals (Safe Signals)">
938 Safe signals is the default behavior, and the old, immediate, unsafe
939 signalling behavior is only in effect in the following situations:
943 =item * Perl has been built with C<PERL_OLD_SIGNALS> (see C<perl -V>).
945 =item * The environment variable C<PERL_SIGNALS> is set to C<unsafe> (see L<perlrun/"PERL_SIGNALS">).
947 =item * The module L<Perl::Unsafe::Signals> is used.
951 If unsafe signals is in effect, then signal handling is not thread-safe, and
952 the C<-E<gt>kill()> signalling method cannot be used.
954 =item Returning closures from threads
956 Returning closures from threads should not be relied upon. Depending of the
957 Perl version and the application code, results may range from success, to
958 (apparently harmless) warnings of leaked scalar, or all the way up to crashing
959 of the Perl interpreter.
961 =item Returning objects from threads
963 Returning objects from threads does not work. Depending on the classes
964 involved, you may be able to work around this by returning a serialized
965 version of the object (e.g., using L<Data::Dumper> or L<Storable>), and then
966 reconstituting it in the joining thread.
968 =item Perl Bugs and the CPAN Version of L<threads>
970 Support for threads extends beyond the code in this module (i.e.,
971 F<threads.pm> and F<threads.xs>), and into the Perl interpreter itself. Older
972 versions of Perl contain bugs that may manifest themselves despite using the
973 latest version of L<threads> from CPAN. There is no workaround for this other
974 than upgrading to the latest version of Perl.
976 Even with the latest version of Perl, it is known that certain constructs
977 with threads may result in warning messages concerning leaked scalars or
978 unreferenced scalars. However, such warnings are harmless, and may safely be
989 L<threads> Discussion Forum on CPAN:
990 L<http://www.cpanforum.com/dist/threads>
992 Annotated POD for L<threads>:
993 L<http://annocpan.org/~JDHEDDEN/threads-1.66/threads.pm>
996 L<http://code.google.com/p/threads-shared/>
998 L<threads::shared>, L<perlthrtut>
1000 L<http://www.perl.com/pub/a/2002/06/11/threads.html> and
1001 L<http://www.perl.com/pub/a/2002/09/04/threads.html>
1003 Perl threads mailing list:
1004 L<http://lists.cpan.org/showlist.cgi?name=iThreads>
1006 Stack size discussion:
1007 L<http://www.perlmonks.org/?node_id=532956>
1011 Artur Bergman E<lt>sky AT crucially DOT netE<gt>
1013 threads is released under the same license as Perl.
1015 CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>
1017 =head1 ACKNOWLEDGEMENTS
1019 Richard Soderberg E<lt>perl AT crystalflame DOT netE<gt> -
1020 Helping me out tons, trying to find reasons for races and other weird bugs!
1022 Simon Cozens E<lt>simon AT brecon DOT co DOT ukE<gt> -
1023 Being there to answer zillions of annoying questions
1025 Rocco Caputo E<lt>troc AT netrus DOT netE<gt>
1027 Vipul Ved Prakash E<lt>mail AT vipul DOT netE<gt> -
1028 Helping with debugging
1030 Dean Arnold E<lt>darnold AT presicient DOT comE<gt> -